US2459297A - Ignition system - Google Patents

Description

Jan. 18, SMITH IGNITION SYSTEM v lnneni'or a zzarian E 5m T attorney Filed Oct. 29, 1945 Patented Jan. 18, 1949 IGNITION SYSTEM Lucian B. Smith, Flint, Mich., assignor to General Motors Corporation, Detroit, Mich, a corporation of Delaware Application October 29, 1945, Serial No. 625,245

This invention has to do with high frequency ignition systems for internal combustion engines and is an improvement on the systems described and claimed in the following prior patents: No.

2,030,228 granted Februar 11, 1936, to D. W. Randolph and O. S. Dufiendack and R. R. W olfe; No. 2,071,573 granted February 23, 1937, to D. W. Randolph and Hector Rabezzana; and No. 2,197,114 granted April 16, 1940, to Hector Rabez zana and W. A. Bychinsky.

The system is characterized by the fact that the rate of voltage rise at the spark plugs is so high that the plugs will continue to function long after they would have been short-circuited by carbon, lead or other combustion chamber deposits had they been supplied with low frequency current. The high frequency discharge is so rapid that there is insufiicient time for short-cir cuiting to take place.

The system is also characterized by distribution of current to the plugs at low voltage together with the provision of a transformer at each of the plugs to raise the voltage to that required to produce the spark in the engine. By this arrangement diflicult problems arising from high voltage distribution, such as flashover at high altitude, deterioration of the wiring harness by Corona discharge, and leakage of current through the capacitance of the ignition cable are avoided. At the same time the ignition circuit may be much more easily shielded so as not to interfere with radio communication.

The improved system, like those disclosed in the patents referred to, embodies a condenser in series with a gaseous discharge tube and the primary of a transformer. The condenser is supplied with energy from a suitable source such as a magneto, preferably having associated therewith a low tension transformer. The condenser stores energy received from the magneto until the voltage of the tube is reached whereupon a high frequency discharge takes place and this, through the plug transformer causes a spark to jump the gap of the plug. In the improved circuit herein disclosed, but one condenser is employed to serve all of the plugs through a suitable distributor, but if preferred individual condensers may be used for each plug.

There is also preferably provided in connection with the magneto a suitable breaker which interrupts the current adjacent the peak of the gen erated current and insures accurate timing of the sparks.

An important feature of the invention consists in the use of a resistance in series with the con- 5 Claims. (Cl. 315255) denser. This resistance limits the charging current to the condenser so that fewer spark discharges take place through the circuit. Its use makes certain that the heavy tube current is not flowing at the time that the connection is broken at the distributor segment and thus increases distributor life. It also prevents unnecessary loading of the secondary of the low tension transformer which might otherwise prevent the flux from building up in the transformer in preparation for the next spark.

Various other features of the invention will be disclosed in connection with the following description in which:

Figure 1 is a diagrammatic view of the ignition system.

Figure 2 shows diagrammatically a modified form of resistance for use in the circuit of Figure 1.

Referring to Figure 1 there is illustrated as the source of energy a magneto of conventional construction consisting of permanent magnets In, each provided with pole pieces l2 and M, the latter being connected by laminated core 56 on which are wound a primary winding P and a secondary winding S. I8 indicates a laminated rotatable inductor driven from the engine in any suitable manner not shown. The permanent magnets I0 are preferably arranged with like poles adjacent the opposed pole pieces I2 so that upon rotation of the inductor magnetic flux flows through the laminated core l6 first in one direction and then in the other, thereby producing an alternating current in the windings P and S. In series with the primary winding P are the contacts of a circuit breaker 20 of conventional construction provided with the usual condenser as shown to reduce sparking. The primary circuit may be provided with a ground connection as indicated at 22. The secondary winding S ma be, but not necessarily, connected at one end to the primary circuit as shown and in series therewith is a resistance R, preferably in the neigborhood of ohms, and condenser C, the latter being grounded to complete the circuit. A condenser having a capactiy of .1 mf. was found adequate in the installation.

In some applications, particularly Where radio interference is a serious problem, it may be desirable to provide a separate ground for secondary S of high tension transformer T and, instead of grounding primary P of said transformer, condenser C and secondary S of the magneto transformer. connect these elements by means of an insulated conductor. This would eliminate the normally heavy grounding currents which flow through the engine frame.

Arranged in the discharge circuit for the condenser C is the distributor D. In series with each of the contacts 24 of the distributor is a tube 26 and the primary P of a transformer T, associated with one of the plugs. For simplicity there is shown the complete circuit between but one of the distributor segments and one of the plugs but it will be understood that in practice a similar circuit is provided for each of the segments.

The secondary circuit consists of the winding S connected at one end to the primary P and to ground as shown and at the other end to the center electrode of the spark plug 28, the other electrode of the spark plug being grounded as usual. I

The system operates as follows:

When the inductor I8 is driven by the engine the reversing flux set up in the laminated core l6 produces an alternating voltage in the transformer coils P and S. The engine-driven interrupter 20 is so timed as to break the primary circuit when the generated current is at or near maximum value. The resulting collapse of the magnetic field associated with the interrupted primary current induces a voltage in the secondary S which is applied to condenser C through resistance R. The condenser serves to store the energy and, before the charge on the condenser has reached the critical value, the condenser is connected through the distributor D to a tube 26 and the primary P of the transformer T. When the condenser voltage reaches the critical value necessary to cause the tube 26 to break down an arc is established between the tube electrodes and a heavy current surge of steep wave front goes through the primary P of the transformer T, inducing a current of high frequency and high voltage in the secondary. winding S which in turn produces a discharge at the spark plug. The condenser C, tube 26and primary P of the spark plug transformer act' as a high frequency generator and, if the secondary of the spark transformer is open, the current oscillates. The frequency of oscillation in a typical installation was approximately one-third to one-half megacycle.

The resistance R limits the charging current to the condenser C so that fewer discharges take place through the oscillatory circuit consisting of the condenser, tube and primary P. The resistance also serves to damp out the current following discharge at the plug to insure that there is no flow at the time the connection is broken at the distributor and this insures longer life for the latter. The resistor likewise limits the current in the secondary S so that no difliculty is encountered in reversing the magnetization of the core in preparation for the next spark.

The magneto equipped with the breaker as described provides voltage characterized by a steep wave front. This characteristic is accentuated by the condenser discharge through the tube. As a consequence the period of time during which the spark plug electrodes are subjected to voltage before a spark occurs is so short that there can be no important leakage of current through paths in parallel with the spark plug gap such as are often provided by conducting deposits of carbon; lead, moisture or other elements on the portion of the insulator projecting into the combustion chamber.

The voltages obtained with this system across such a shunting resistance are exceptionallyhigh as compared with those obtained with a magneto ignition system of conventional type. Thus, while with the latter six kilovolts could be obtained across a leakage path having 135,000 ohms resistance, with the system here described six kilovolts have been obtained across a leakage path having but 6,000 ohms resistance.

It is characteristic of the conventional ignition stantially constant at all speeds in excess of that required for starting. Thus, in the case of a particular installation the speed at which the present system began to function was approximately 25 R. P. M. of the engine and the voltage obtained at the plug at that engine speed was substantially the same as the voltage obtained at top engine speeds. Similarly, while in a conventional magneto ignition system the voltage at the plug varies somewhat with the temperature, in the present system the voltage output" is but slightly affected by increased temperature.

Engine tests have indicated that the wearfon spark plug electrodes is considerably less with the ignition system here disclosed than with conventional systems. This advantage no doubt results from the steep-fronted secondary currentan'd the very short time it is effective;

It has been discovered in' tests of engines equipped with this ignition system that it isdesirable to employ a variable resistance at R. The difficulty with a fixed resistance is that if'it is high enough to give the desired'damping' at high speeds-e. g. speeds of approximately 200 sparks per second and higher'so as to' prevent missing, the speed at which the engine must be driven to start it is increased and this isundesirable. The difficulty may be overcome by :employing a'special resistance which increases with engine speed. One way of accomplishing this is to employ wire, the resistance of which increases rapidly with increase of temperature, for example wire having a temperature coefficient of resistance of about .0045 ohm per ohm per-degree Centigrade in placev of standard resistance wire having a temperature coefficient of resistance of about .0002 When starting an engine with a variable resistance of this sort. the resistance. is cold and has a low value, for example approximately 150 ohms, but as the engine speed increases the current through the'resistor increases causing a temperature rise and consequent. resistance rise to about 300 or e00 ohms, at 3600 R. P. .M. The increased resistance at higher speeds insures against missing of the engine through ignition failure. I I g Another wayof accomplishing this result is disclosed in Figure 2 in which there is shown'a speed controlled resistance R. which may be substituted for resistance R of Figure 1. Here resistance R. consists of two parts R2 and Rs; 80

indicates a circuit shunting resistance R3 and containing switch 8| which is adapted to'fbe opened in any suitable manner by the operation of centrifugal force as the speed increases thereby increasing the resistance in series'with condenser C. Merely for purposes of illustration, I have indicated at 82 a conventional type of centrifugal,

governor connected to the switch by link 84 and adapted to openit when the governor is'driven at sufiicient speed. Obviously the mechanism could readily be designed to increase the resistance by several stages as the speed increases.

The circuit is susceptible of many other modi- With the system here fications. For example, other types of magnetos may be employed, or, if desired, a generator not employing permanent magnets may be used. In some instances it may be satisfactory to employ a battery, inductance and interrupter in place of the generator as disclosed in the Randolph and Duffendack patent referred to. However, a source of alternating current is particularly desirable as the reversal in direction of current flow lengthens the lives of tubes and contacts. While the magneto illustrated is of the type embodying a built in transformer, if desired the transformer may be an independent unit or may be omitted entirely, and the primary voltage which appears across the primary condenser may be applied directly to the tube and high tension coil.

The distributor may be of conventional construction and is preferably of the type in which the brushes make sliding contact with the segments connected to the plugs.

The tube is preferably of the construction described and claimed in application Serial No. 589,392, filed April 20, 1945, by Ralph S. Mitchel, consisting essentially of low Work function electrodes preferably of a nickel barium composition in an atmosphere of inert gases, preferably a mixture of neon, argon and helium. While a particular design of tube is illustrated, it will be understood that the design is susceptible of considerable variation.

There may be considerable variation in the arrangement of parts on the engine. For example, it has been found convenient in one installation to mount the tubes on the distributor terminals, the distributor itself forming a part of the magneto assembly. In some instances it may be found satisfactory to employ a single tube rather than one for each spark plug and in such case the tube would be inserted in series with the condenser and in advance of the distributor.

Accurate timing is insured by employing an interrupter in the primary circuit of the magneto. However in some applications it may be found possible to dispense with the interrupter and rely upon the action of the tube to secure the desired timing.

If desired a separate condenser could be provided for use with each plug, but this complicates the system and offers little advantage.

It may prove desirable to tune the circuits for optimum impedance, particularly the discharge circuits of the condenser.

I claim:

1. An ignition circuit comprising a source of alternating current, a step-up transformer having its primary in series with the source, a damping resistance and a condenser in series with the secondary of the transformer, a second stepup transformer, and a discharge circuit for the condenser comprising a distributor, an electrical discharge tube adapted to break down upon application of a predetermined voltage, and the primary of said second step-up transformer, the secondary of said second transformer having a spark discharge device in series therewith.

2. In an ignition system for internal combustion engines of the type in which pulsating electric current is supplied from a source to a condenser to cause it to periodically and at a frequency determined by engine speed build up to a voltage sufficient to cause break down of an enclosed spark gap in series with an ignition device to produce ignition, a resistance in series with said source and condenser and adapted to damp out oscillations of current upon break down of said gap, and means for increasing said re sistance with increase in speed of the engine.

3. In an ignition circuit, a condenser, means for periodically charging the condenser comprising a source of pulsating current and a charging circuit between the source and the condenser including a damping resistance, and a discharge circuit for the condenser comprising in series a distributor and a plurality of parallel circuits adapt-ed to be successively brought into series with the condenser by the distributor, each of said parallel circuits comprising a discharge tube adapted to break down upon application of a predetermined voltage and the primary of a stepup transformer, the secondary of said transformer having a spark discharge device in series therewith, and said damping resistance being of such value as to damp out the condenser discharge in sufficient time to permit re-charging of the condenser between spark discharges.

4. In an ignition circuit, a condenser, means for periodically charging the condenser comprising a source of pulsating current and a charging circuit between the source and the condenser including a damping resistance, and a plurality of discharge circuits for said condenser each comprising a discharge tube adapted to break down upon application of a predetermined voltage and the primary of a step-up transformer, the secondary of the transformer having a spark discharge device in series therewith, and a distributor for directing discharge of said condenser through each of said discharge circuits in succession, said damping resistance being of such value as to damp out condenser discharge in sufiicient time to permit re-charging of the condenser between spark discharges.

5. In the circuit as defined in claim 4, said resistance having a relatively high positive temperature coefficient of resistance.

LUCIAN B. SMITH.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS

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